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Old 8th Sep 2017, 11:18 am   #1
mole42uk
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Default Selective insulation rot?

I'm working on an old piece of equipment and discovered that the wiring to the mains transformer had flaking insulation.

On further investigation, of the wires into the transformer, only the ones with blue insulation were flaking, and the copper inside was almost completely destroyed. So, red, green and black pigment is okay, blue rots the copper and itself. The good insulation feels like rubber.

Has anyone any ideas aout the chemistry of this?
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Old 8th Sep 2017, 11:49 am   #2
Phil G4SPZ
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Default Re: Selective insulation rot?

I sometimes re-use salvaged rubber-insulated wire in good condition during restorations. When stripping back apparently perfect rubber insulation, it's common to find some sort of minor corrosion to the stranded cores beneath. Unless thoroughly cleaned and abraded, the wire resists soldering.

I've not personally noticed any colour-related variations. Perhaps your blue wires ran at a higher temperature and thus accelerated the corrosion? It's perfectly possible that rubber wiring insulation came in different qualities; some types seem to perish badly whereas other (older) types seem to last indefinitely. An interesting topic!
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Old 8th Sep 2017, 11:55 am   #3
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Default Re: Selective insulation rot?

Never noticed it myself, but we've had numerous reports of colour-dependent deterioration of VIR insulation, both in house wiring and in radios/TVs on the forum over the years.

It may, of course, be nothing to do with the colouring agents per se, as the rubber may have been made by different companies and would obviously have come from different batches etc.
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Old 8th Sep 2017, 12:19 pm   #4
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Default Re: Selective insulation rot?

I've had many occasions when the yellow insulation has totally rotted and flaked off but other colours in the same loom are OK.
Vulcanised rubber is funny stuff, just look at those brown jelly grommets that go most peculiar with crispy edges.
Some VIR cables in house wiring are still serviceable and test out OK whilst others fall apart as soon as disturbed. There used to be a marked difference in the deterioration between the red and the black insulation with some makes I remember.
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Old 8th Sep 2017, 7:55 pm   #5
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Default Re: Selective insulation rot?

I had similar on a WWII-vintage Type 'F' Mk:II field-telephone. The wiring in the loom was VIR, multi-coloured, and, whilst electrically intact, the insulation was extremely gooey to the touch. I can't say I noticed any degredation specific to one particular colour, though, but there was some corrosion on the copper within.
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Old 8th Sep 2017, 7:58 pm   #6
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Default Re: Selective insulation rot?

These are four cables twisted together. The ones that are okay feel like rubber insulation, perfectly sound. The insulation on the blue one has flaked off, no gooeyness, it's all flaky and dry, but the copper stranded wire inside is corroded to nothing in places.
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Old 8th Sep 2017, 8:15 pm   #7
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Default Re: Selective insulation rot?

Very common on LOPT lead out wires insulated with that brown organic sleeving that turns to toffee apple consistency. I once had it on every wire rotted in the middle of the sleeving. [Vidor CN369A 9" television receiver 1947] J.
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Old 8th Sep 2017, 8:19 pm   #8
The Philpott
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Default Re: Selective insulation rot?

Might this be a Copper Phthalocyanine pigment? If there is sulphur in the rubber as well, i can imagine all sorts of alchemy going on in the medium to long term.

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Old 8th Sep 2017, 10:41 pm   #9
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Default Re: Selective insulation rot?

Vulcanising of latex rubber is due to sulphur, so that seems to be highly likely.
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Old 8th Sep 2017, 10:59 pm   #10
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Default Re: Selective insulation rot?

In the Tektronix 7000 series mainframes and plugins, insulation displacement "harmonica" connectors were used - kind of Molex of today.

These were colour coded, and the one that causes problems is the blue ones. They simply crumble away if you handle them and expose the female crimped on terminals. All other colours are fine.

This line of scopes was introduced in 1968 and ran until about 1991. But the problem with blue connectors seems to be only present in older 'scopes.

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Old 8th Sep 2017, 11:43 pm   #11
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Default Re: Selective insulation rot?

Seems very common on mains leads, sometimes red is perfectly serviceable while black has gone hard and crumbly, or vice versa
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